NEW YORK, NY - OCTOBER 08: Gigi Hadid (L) and Bella Hadid attend the Global Lyme Alliance "Uniting for a Lyme-Free World" Inaugural Gala at Cipriani 42nd Street on October 8, 2015 in New York City. (Photo by Dimitrios Kambouris/Getty Images for Global Lyme Alliance)

When someone is struggling with a chronic illness, sometimes the signs are obvious. What we often forget, though, is that sometimes chronic illness is invisible. Someone might look and act fine, but be struggling with a debilitating illness. We saw that so clearly in the Real Housewives of Beverly Hills, where Yolanda Hadid, mom to models Bella and Gigi, aired her personal battle with Lyme disease while her cast mates cast doubt on whether or not she was actually sick.

"This book marks such an incredible day for you. I have seen you in bed, struggling for the past 5 years of our lives, thinking to myself 'why would this happen to such an angel?'" Bella wrote. "I would cry to myself knowing that there was nothing I could do to help and that there were people going against you and what you are/were going through."

On season six of the Real Housewives of Beverly Hills, Yolanda's cast mates questioned whether or not her illness was actually as bad as she said it was. Yolanda spent much of the season in bed and in doctors' offices, searching for treatment and cure for the disease. Housewife Lisa Rinna questioned whether Yolanda was sick or suffering from Munchausen syndrome, a mental condition in which a person repeatedly acts like they are sick when they are not.

The problem with Lyme disease is that even after it's treated, symptoms can persist. Known as Post-treatment Lyme Disease Syndrome, doctors aren't really sure what causes the tick-borne illness to linger in some people even after the typical round of antibiotics used to treat the initial infection. Symptoms of chronic Lyme include nervous system issues, muscle and joint problems, heart and circulation complications, digestion issues, reproductive problems and skin issues. These symptoms can come and go.

Because you can't really see these complications, many of the Beverly Hills Housewives wrongly doubted Yolanda, some saying she used the disease as an excuse to get out of social obligations. Yolanda isn't the only one whose illness was doubted, though, Bella revealed.

"I have been struggling with Lyme and other co-infections for the past two years which has debilitated me from riding my horses, going to school, and struggling with work which are all the things I am so passionate about," Bella wrote. "You are the only person that would believe me and understand what I was going through."